1. Field of the Invention.
The present invention pertains to methods and apparatus for detecting localized pipeline deformations. More particularly, this invention pertains to the use of attitude-sensing techniques to detect such deformations.
2. Description of the Prior Art
The monitoring of pipeline flexure over time provides a useful aid to protection of a surrounding ecological environment. Changes in localized pipeline curvature can result from variable soil, temperature and exogenous forces (such as earthquake) and such changes over time provide an advance indication of pipeline stressing and fatigue that could result in rupture. Often early detection of problem areas along the length of a pipeline allows repair, such as the "springing" of a flexed segment of pipeline, to be affected at minimal cost. By accurately locating points of excessive flexure, preventive maintenance can often be rapidly effected, avoiding additional damage to the pipeline.
In Arctic environs, such as that of the Alaska pipeline, non-static conditions along a 900 mile length can result from the flow of oil at temperatures ranging from 20 degrees to 145 degrees Fahrenheit through a pipeline that is surrounded by permanently frozen (permafrost) soil. Such a flow can thaw subsoil in proximity to the pipeline and/or to various supporting structures to sink or shift pipeline sections into attitudes that result in localized curvatures in excess of desired tolerances. The cold temperatures associated with liquified gas cause similar pipeline problems.
It is, therefore, highly desirable to detect excessive curvatures within pipelines prior to failure and a number of systems have been proposed for this purpose. Two methods have attained primary significance in the measurement of curvatures along relatively long pipelines. The first method involves the welding of a plurality of regularly spaced bars along the pipeline. The bars extend through the surface of the soil, providing an indication of the height of the pipeline section to which it is welded. Survey techniques determine changes in the heights of the bars over time, indicating changes of curvature at preselected points along the pipeline. This method is relatively expensive and, due to the employment of manual survey techniques, subject to human error. Additionally, the positioning of bars at a plurality of points along the pipeline produces data that is inherently non-continuous. Thus, a significant number of bars must be utilized to avoid the probability of non-detection of non-monitored points between bars. Additionally, severe weather conditions hamper efforts to survey accurately.
The other method primarily employed is in accordance with the disclosure of U.S. Pat. No. 3,882,606 of Kaenel et al for "Method and Apparatus for Measuring Curvature and Curvature Variations in Pipelines and the Like". In that patent there is disclosed a pipeline "pig" having a plurality of wheels engaged to its opposed sides. The wheels, which act as sensing wheels, travel the inner surface of the pipe. Pipeline curvature is determined by comparison of the distances traversed by wheels located at opposite sides of the pig. In the event that a curvature exists in a section of the pipeline, it will be reflected in a differential between the lengths traversed by the opposed wheels. This method is subject to a number of error-inducing factors including wheel slippage, pipe irregularities, and tracking errors. Additionally, unequally distributed wrinkling along the inner surface of the pipe can effect the accurate measurement of curvature by this method.
Thus, it is desirable to provide new apparatus and methods for measuring localized curvatures throughout the length of a pipeline.